A multiple cell AC plasma panel is one type of gas discharge panel in which a gas medium, typically comprised of a mixture of two or more gases at a suitable gas pressure, is disposed in a transparent chamber or envelope. A multiplicity of electrodes are positioned on both sidewalls of the envelope and each electrode is normally coated with a dielectric material so that an ionized current flows therebetween during the operation of the panel creating a light pulse.
The dielectric layers disposed on the electrodes are significant because the current created by the ionized gas is prevented from actually reaching the electrodes during each half cycle of the operation. As is known, once the arc of ionized gas is struck at a particular discrete site, charges in opposition to the current flow collect on the dielectric surfaces creating an EMF opposing further current flow during that half cycle. During the next half cycle, the process is reversed and charges of opposite polarity build up on the dielectric surface areas that resists electron flow during the next half cycle.
In the construction of an AC plasma panel the multiplicity of electrodes are often positioned in an orthogonal configuration forming a matrix of rows and columns which can be readily addressed to display various types of data. Although the orthogonal configuration is probably the best known electrode arrangement, the electrodes could be arranged in any desired format which might be more convenient for displaying certain types of video data, such as circular electrodes in conjunction with radial electrodes for displaying radar video data and so forth.
A particular problem associated with the use of AC plasma panels, as well as many other solid state panels, is that plasma panels are basically bistable devices, i.e. there are only two modes--on or off. Because of this characteristic, obtaining gray scale variation of the individual discrete areas in the panel has been difficult and a number of different techniques have been used to create the appearance of intensity variations at each discrete panel site, or, as it has become known, gray scale panel operation. Some of the known techniques include:
(1) Multiple sites per resolution area--a plurality of discharge sites are used in each resolvable elemental area and intensity variation is created by increasing or decreasing the number of sites turned on. One such device is disclosed in U.S. Pat. No. 3,886,403 issued May 27, 1975 to Owaki et al for BRIGHTNESS MODULATION SYSTEM FOR A PLASMA DISPLAY DEVICE. In the system described by Owaki et al a single picture element is made up of a plurality of discharge cells and the perceived brightness is proportional to the total number of discharges of all the cells within a given period. The illusion of brightness is created by lighting that fraction of those cells in each group which correspond to the average instantaneous level at that point on the incoming video signal. In essence, the brightness of the individual area is proportional to the number of discrete elements turned on in that area. A problem with this particular technique is that each area requires a number of lightable sites, that number corresponds to the number of intensity levels, and hence the manufacturing cost is dramatically increased due to the total number of electrodes required for a given resolution. Another way of looking at this approach is that the technique achieves the impression of brightness variation at the expense of resolution.
(2) Amplitude variation of sustainer pulses--a trigger pulse initiates a sequence of discharges which decrease in amplitude until they eventually die out, the average brightness of the light emitted being controlled by varying the magnitude of the trigger pulse hence the decay time. Both U.S. Pat. No. 4,067,047 issued Jan. 3, 1978 to W. Ryan for CIRCUIT AND METHOD FOR GENERATING GRAY SCALE IN GASEOUS DISCHARGE PANELS and U.S. Pat. No. 4,002,828 issued Jan. 11, 1977 to J. Nolan for METHOD AND CIRCUIT FOR GENERATING GRAY SCALE IN GASEOUS DISCHARGE PANELS describe essentially identical techniques for obtaining gray scale variation at each discharge site in a plasma panel. A trigger pulse is applied across the electrodes at a particular site and initiates a discharge sequence which persists for a known period depending on the initial magnitude and duration of the trigger voltage.
(3) Time or duty cycle modulation--each discrete site can be turned on at a selected time within a given period and the intensity is varied by controlling the relative on time. One such system is disclosed in U.S. Pat. No. 3,863,023 issued Jan. 28, 1975 to L. Schmersal et al for METHOD AND APPARATUS FOR GENERATION OF GRAY SCALE IN GASEOUS DISCHARGE PANEL USING MULTIPLE MEMORY PLATES. A multiplicity of memory planes equal in number to the number of gray level ranges to be reproduced is provided. Each memory plane has the same number of storage areas as the number of storage and discharge areas that the display panel has. An image is scanned and the picture elements are separated into a finite number of shades of gray, equal in number to the number of memory planes provided. Each memory element is stored in its proper sequence in a memory plane associated with the shade of gray that the picture element corresponds to and the contents of the memory plane are then sequenced into the panel to reproduce the original picture. A problem with this technique is that the number of memory planes required is equal to the number of gray shade levels so that, for example, an eight gray shade system requires a memory with eight times the number of picture elements in the display. In addition, all of the elements of the panel are periodically extinguished so that the entire write cycle, which requires addressing each element of the panel individually, must be accomplished within one flicker period, or approximately sixteen milliseconds. For example, if a sustainer frequency of approximately 60 kilohertz is employed. (This approaches the upper possible frequency limit for a plasma panel because of the physical constraints of plasma initiation and decay) and this sustainer frequency is combined with an optimum addressing system so that a discrete element is addressed on each sustainer cycle; then, the maximum number of picture elements in the panel would be limited to approximately 1,000. In general, this size of a display is too small for a large raster, such as a TV raster or the like.
Also of general interest in this latter category is U.S. Pat. No. 3,975,661 issued Aug. 17, 1976 to Kanatani et al for DRIVING METHOD FOR A THIN-FILM ELECTROLUMINESCENT ELEMENT OF A THREE LAYER CONSTRUCTION: U.S. Pat. No. 4,021,607 issued May 3, 1977 to Amano for VIDEO DISPLAY SYSTEM EMPLOYING DRIVE PULSE OF VARIABLE AMPLITUDE AND WIDTH.